The present invention relates to polishing pads for chemical mechanical polishing (CMP), and in particular relates to a polishing pad having optimized grooves.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting, and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP).
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize substrates such as semiconductor wafers. In conventional CMP, a wafer carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the wafer, urging it against the polishing pad. The pad is moved (e.g., rotated) relative to the wafer by an external driving force. Simultaneously therewith, a chemical composition (xe2x80x9cslurryxe2x80x9d) or other fluid medium is flowed onto the polishing pad and into the gap between the wafer and the polishing pad. The wafer surface is thus polished and made planar by the chemical and mechanical action of the polishing layer and slurry.
In CMP, planarity and uniformity of the wafer surface are paramount. Accordingly, CMP systems are typically configured to provide orbital and/or oscillatory motion of the wafer to average out variations in instantaneous local polish rate. It is known that pad and wafer rotation speeds can be combined in a way that, over time, results in each point of the wafer surface being exposed to the same range and mean value of relative pad velocity. Such an arrangement is described in the article by D. A. Hansen et al, entitled xe2x80x9cCharacterization of a Multiple-Head Chemical Mechanical Polisher for Manufacturing Applicationsxe2x80x9d, Proceedings of the 1 st International CMP-MIC, February 1996, which article is incorporated herein by reference.
The averaging mathematics for the wafer and pad rotations presume that the polishing layer is homogeneous with respect to radial position. However, where the polishing layer includes certain types of grooves (e.g., concentric circles, Cartesian grids, fixed-width radii, or combinations of these), the polishing surface area per unit pad area can vary as a function of pad radius.
FIG. 1A is plot of a standard prior art radial groove pattern, such as described in U.S. Pat. No. 5,177,908. FIG. 1B is a plot of the circumference fraction grooved CF as a function of pad radius R for the radial groove pattern of FIG. 1A. For purposes of this application, the circumference fraction grooved CF is as follows:
(Portion of circumference at a given radius that lies across any groove)/CF=(Full circumference at the given radius)
Note: If CF is constant as a function of radius, then the fractional area of the pad that is grooved (or ungrooved) at a given radius is also constant as a function of radius.
With continuing reference to FIG. 1A, since the number and width of the grooves is fixed, the total grooved length along a circumference is the same regardless of radius. Thus, as shown in FIG. 1B, CF decreases as the distance from the center increases, with the value of CF near the outer edge of the pad being many times smaller than that near the center.
FIG. 2A is a plot of a standard prior art concentric circular groove pattern. FIG. 2B is a plot of the circumference fraction grooved CF as a function of pad radius R for the concentric circular groove pattern of FIG. 2A. In this case, CF is unity at any radius that falls within a groove, and zero at any radius that does not. The area fraction grooved is thus a sharply changing function of radius.
FIG. 3A is a plot of a standard prior art Cartesian grid groove pattern with equal pitch in both coordinate directions. FIG. 3B is a plot of CF as a function of pad radius R for the Cartesian grid groove pattern of FIG. 3A. Note that CF decreases with increasing radius until a new set of grid lines is crossed, at which point the fraction sharply increases. At larger values of radius, even small increments in radial distance cross additional grid lines, so that CF is a highly irregular function. At large radius values where CF begins to asymptote, there is significant (i.e., over 50%) variation in the polishing area per unit pad area.
FIG. 4A is a plot of a standard prior art spiral groove pattern, such as disclosed in U.S. Pat. Nos. 5,921,855 and 5,690,540 (the ""540 Patent). FIG. 4B is a plot of CF as a function of pad radius R for the spiral groove pattern of FIG. 4A. Note that CF decreases with increasing radius because the spiral curve does not grow in exact proportion to the radius.
Accordingly, there is a need for a polishing pad with grooves that properly account for the mutual rotations of the wafer and polishing pad.
An aspect of the invention is a polishing pad useful for chemical mechanical planarization, the polishing pad having a polishing layer for planarizing substrates, the polishing layer comprising: a radius that extends from a center of the polishing layer to an outer perimeter of the polishing layer; one or more continuous grooves formed in the polishing layer and extending inward from the outer perimeter of the polishing layer; and a circumference fraction grooved (CF) in an area extending from the outer perimeter of the polishing layer a majority distance to the center of the polishing layer, CF being that portion of circumference at a given radius lying across the one or more continuous grooves divided by full circumference at the given radius, and wherein CF remains within 25% of its average value as a function of the polishing layer radius in the area extending from the outer perimeter of the polishing layer the majority distance to the center of the polishing layer.
In another aspect of the invention, the one or more continuous grooves start at a base radius and extend to an outer perimeter of the pad. Alternatively, the one or more continuous grooves start at a starting radius between the base radius and the outer perimeter, and extend to the outer perimeter.
Another aspect of the invention is a method of planarizing a wafer surface. The method of chemical mechanical planarizing a substrate comprises the steps of: introducing a polishing solution to a wafer; rotating the wafer with respect to a polishing pad, the polishing pad having a polishing layer, and the polishing layer comprising: i) a radius that extends from a center of the polishing layer to an outer perimeter of the polishing layer; ii) one or more continuous grooves formed in the polishing layer and extending inward from the outer perimeter of the polishing layer; and iii) a circumference fraction grooved (CF) in an area extending from the outer perimeter of the polishing layer a majority distance to the center of the polishing layer, CF being that portion of circumference at a given radius lying across the one or more continuous grooves divided by full circumference at the given radius, and wherein CF remains within 25% of its average value as a function of the polishing layer radius in the area extending from the outer perimeter of the polishing layer the majority distance to the center of the polishing layer; and planarizing the wafer with the polishing pad and the polishing solution.